Assuming the battery is in good condition, the open circuit voltage can
be a fair indicator of the charge of a battery if it has been rested and
the surface charge removed.

The specific gravity is a better indicator of charge, but again, the
battery must be in good condition.

In either the SG or the voltage test, a damaged battery may indicate
full charge, but be useless. If a plate has dropped, a battery could be
fully charged but not capable of delivering any current. If the water
is low (or in the case of 'service free' batteries, it is common for the
water to get so low that it just barely touches the bottom of the
plates), the voltage is fine, but it won't light a light bulb.

So, the third test is neccessary occasionally to check the 'health' of
the battery. This will tell you that the battery is capable of
delivering sufficient current. Then once you know you have a healthy
battery the SG or rest voltage will tell you the charge state.



Charles T. Low wrote:
Doug,

Thanks, very helpful.

So, does this seem valid to you? - An accurate voltage reading is a good
indicator of a battery's condition, but specific gravity is better, and a
slow load test (to 50%) is the best? Or are they testing different things,
all of which are important? Could it do well on a load test without good
voltage or sg's?

And, what use is a commercial load tester, then? Why do they make them? Is
it better than an sg but not as good as a slow load test?

Thanks again. Charles

P.S. I corrected the spelling of "betteries" - couldn't take it any more!

====

Charles T. Low
- remove "UN"
www.boatdocking.com
www.ctlow.ca/Trojan26 - my boat

====

"Doug Dotson" wrote in message
...

The discharge time of a deep cycle battery is influenced by many
things other than SG. The condition of the plates etc all come into
play. While one may be able to draw heavy current for a brief
period that does not necessarily translate into good low power
performance. A long term low current test tells the actual story
since it is how the battery is being used in normal use. The SG
gives the state of charge from a theoretical sense as does the voltage.